The notion of using a liquid solvent to extract lower alcohols, e.g., ethanol, from an aqueous solution has been pursued since the early 1980s. For example, in 1984, Munson and King published “Factors Influencing Solvent Selection for Extraction of Ethanol from Aqueous Solutions,” Industrial and Engineering Chemistry Process Design and Development, 23, p 109-115. Munson and King examined solvents and solvent mixtures for the extraction of ethanol from dilute aqueous solutions. Results were tabulated on the basis of capacity, as represented by the distribution coefficient, and selectivity, as represented by the separation factor. Munson and King showed that an increasing distribution coefficient generally correlates with a decreasing separation factor. Thus, as the solvent become more effective for extracting ethanol, the solvent, unfortunately, becomes less effective for rejecting the water.
Previously disclosed methods of using an oil to extract ethanol from a dilute aqueous solution have proven to be energetically and economically inefficient. For example, Metha and Fraser, “A Novel Extraction Process for Separating Ethanol and Water,” Industrial and Engineering Chemistry Process Design and Development, 24, 1985, p 556-560 detail a method to use light paraffin oil to extract ethanol from water. Their method leverages the ternary phase behavior of ethanol-water-paraffin oil system. The proposed process scheme requires process temperatures in the range from 30° C. to 115° C. The report does not provide the optimum process conditions. Ethanol's boiling point is 78° C. Furthermore, in order to have favorable energy input into the process, the process requires that paraffin oil travel with the discharged ethanol. Because paraffin oil is more valuable than ethanol, it is not clear that the proposed process has an economic advantage.
Numerous published methods for the extraction of ethanol require a distillation step to remove ethanol from water, which is energetically and economically inefficient, and an unnecessary additional step. For example, U.S. Pat. Nos. 4,409,406; 4,865,973; 4,770,780; 5,036,005; and 5,215,902 each disclose processes for the extraction of ethanol that require a distillation step to remove ethanol from water.
Others have also proposed using carbon dioxide as a primary extractant of ethanol from an aqueous solution. However, these methods are limited by the distribution coefficient between ethanol-water and CO2 that has been measured to be on the order of 0.1 by numerous researchers, e.g., Krukonis (FIG. 8.11, p. 173, McHugh, M., Krukonis, V., Supercritical Fluid Extraction, 2nd Ed., Butterworth-Heinemann, 1994). These processes have no energy advantage over a traditional binary distillation process. See, for example, U.S. Pat. Nos. 4,842,693; 5,160,044; and 4,770,780.